(a) Field of the Invention
The present invention relates to a photographic lens system having a field angle of 70.degree. to wider and designed for use with a compact camera (adopting for a photographic lens and a view finder which are designed as separate units) having a relative aperture of F/3.5.
(b) Description of the Prior Art
As lens systems for use with a compact camera, there have conventionally been known triplet type, Tessar type and variant types thereof. However, these conventional photographic lens systems had field angles on the order of 60.degree. at maximum. Recently, there are known the so-called telephoto type of lens systems having short total lengths for compact cameras comprising a positive front lens group and a negative rear lens group. These lens systems are also designed for field angles on the order of 60.degree. to 65.degree..
Generally, it is important to favorably correct curvature of field for obtaining a lens system having a large field angle. In order to correct curvature of field, it is necessary to minimize Petzval sum, and for this purpose in turn, it is necessary to arrange a negative lens component in the lens system. As this nagative lens component has a lower refractive index, Petzval sum is smaller and curvature of field is correted more favorably. In a lens system for use with compact cameras, positive lens components are arranged on the front side of the lens system for shortening the distance as measured from the first lens surface to the film surface (hereinafter referred to as "total length of lens system"). In order to obtain a shorter total length of lens system, these positive lens components must have higher refractive power, which allows negative spherical aberration to be aggravated. Therefore, it is general to arrange a negative lens component between these positive lens components so as to correct curvature of field by minimizing Petzval sum and, at the same time, correct the negative spherical aberration. The function to correct this negative spherical aberration is more effective when the object side surface of the negative lens component is concave toward object to be photographed and has smaller radius of curvature since the paraxial ray incident on this surface has large angle of incidence at higher level on the surface. Therefore, a lower refractive index of this negative lens component is more advantageous for correcting curvature of field and spherical aberration. When this surface has a small radius of curvature, however, it produces coma since angle of incidence is largely different between the upper and lower rays of the offaxial ray incident on this surface at large field angle. Therefore, coma is aggravated abruptly at larger field angle. For this reason, it has hitherto been difficult to design a lens system having a field angle of 70.degree. or larger for use with compact cameras.
As a conventional example having a lens composition similar to that of the lens system according to the present invention, there has already known the one disclosed by Japanese Unexamined Published Patent Application No. 95207/81. This lens system is designed compact but has a field angle on the order of 60.degree. to 65.degree..